This invention relates to apparatus and method for sensing and processing incoming electromagnetic radiation signals to simultaneously determine frequency and directional angle-of-arrival of such signals at an antenna array for various surveillance purposes.
Various techniques have previously been employed for determination of frequency and angles-of-arrival of a microwave signal through use of electronic circuits. One known method of calculating the angle-of-arrival of a microwave signal is to sense such signal with two coplanar antennas. The phase-difference between antenna receptions is dependent on the angles-of-arrival and frequency of the incoming microwave signal. Thus, with knowledge of such phase-difference and the signal frequency, the angles-of-arrival can be calculated. Currently known electronic methods determine this phase-difference and frequency by using channelized receivers which require multiple phase-matched channelizer banks with phase comparison circuitry at the outputs of the channel filters. Such techniques require a large volume of electronic circuitry, with accompanying power requirements and weight problems.
An alternative approach to the foregoing signal sensing techniques reside in the use of acousto-optic devices which require at least one acousto-optic cell per antenna with the deflected light from each acoustic-optic cell combined by optical interferometry. Each cell is driven with a signal from a different antenna and the output of each cell reflects the characteristics of the signal of the respective antenna. Frequency is determined by the angle of the deflection of the light emerging from the acousto-optic cells, and the phase of the light from each cell contains data on the angle of arrival of the microwave signal. In order to extract such data from the phase information of the deflected light from each cell, the light is combined interferometrically. The interferometric recombination involved may be accomplished by arranging the two acousto-optic cells in separate arms of an interferometer, such as a Mach-Zehnder interferometer, or by arranging the acousto-optic cells as elements of a diffraction array. The disadvantage of the foregoing acousto-optic cells approach is the redundancy of using two cells if one will suffice In the case of the use of a diffraction array, a difficulty arises in keeping track of a complicated light diffraction pattern as the phase of acoustic signals change with the variation in phase of the microwave signals that drive the respective cells.
U.S. Pat. No. 4,503,388 to Zehl et al. illustrates, for example, an arrangement of a type commonly used for determining frequency of incoming RF signals by passing laser light through a Bragg cell for diffraction in response to a RF signal stimulating the cell. Another spectrum analyzer system is shown in U.S. Pat. No. No. 4,636,718. According to U.S. Pat. No. 4,644,267 to Tsin et al., an arrangement is disclosed for measuring the angle of arrival of a signal on a two quadrant antenna which receives the signal with a phase difference. A signal frequency measuring receiver is therefore employed for cueing the frequency of the phase comparison unit.
It is therefore an important object of the present invention is to provide a more efficient microwave signal processing apparatus and method that simultaneously determines frequency and directional angle-of-arrival of incoming electromagnetic signals impinging on an antenna array.
It is a further object of the invention to more efficiently determine angles-of-arrival and frequency of active radar signals.
It is yet another object of the invention to passively sense signals for characterization and identification of communication systems or radar systems.